The present invention relates to a radial tire for a passenger car and particularly to a radial tire for a passenger car having a low rolling resistance and more particularly it is directed to provide a tire tread which is well balanced in both performances of the rolling resistance and the wet grip.
Recently effective use of energy resources is requested throughout the world and it is also necessary to save energy resources in the use of tires. Namely, we must contribute in reducing the consumption of energy resources used by cars. It is therefore of an urgent necessity in the tire manufacturing industry to develop techniques to produce energy saving tires.
Rolling resistance of a tire is mainly caused by the inner friction of the materials of which the tire is composed, namely, the materials of tread, sidewall, carcass, and inner liner. Most of all, a tread affects the rolling resistance of a tire. Accordingly, it is most effective for obtaining low rolling resistance of a tire to reduce the energy loss caused by the compression, bending, and shearing of the rubber compound of a tread.
But it is a well-known fact that a tread composition having good wep grip performance is inforior in the rolling resistance and the two performances are incompatible with each other.
There is disclosed in the Japanese patent application laid open under No. 55-99403 that low rolling resistance as well as a high wet grip performance can be obtained in a tread rubber wherein the resilience (the percent ratio of the amount of energy used for recovering from a deformed condition to the amount of energy used for causing a sudden deformation) is more than 65% as to the tread base rubber and is less than 60% as to the tread cap rubber. In another Japanese patent application laid open under No. 55-106803, there is disclosed that the compatibility between the contradictory performances of wet grip and rolling resistance can be achieved if the loss tangent (tan .delta.) of a cap tread rubber layer has a specific value between 0.25 and 0.5 and that of a base tread rubber layer has also a particular value less than 0.2.
In the above two patent applications, the resilience of a tread cap rubber is less than 60% or the loss tangent (tan .delta.=E"/E', namely, the ratio of the loss elastic modulus (E") to the storage elastic modulus (E')) is somewhere between 0.25 and 0.5. However, the use of the said rubber composition as a tread cap rubber gives a bad effect on the performance of the rolling resistance of a tire since the volume of a tread cap rubber becomes bigger than that of a tread base rubber so that the rolling resistance is greatly affected by the tread cap rubber. It can be said that this has resulted from the fact that consideration has been taken into the improvement of wet grip performance more than that of the performance of rolling resistance.